There is increasing attention given to amino acids as neurotransmitters in mammalian central nervous system. Among those currently under investigation gamma aminobutyric acid (GABA), glycine, glutamic acid, and taurine possess several of the characteristics considered requisite for a neural transmitter substance. Each of them are present in nerve endings together with the enzyme responsible for their synthesis. There is an efficient mechanism for termination of their action, and their effect on neuronal membranes is identical with that of the natural transmitter. Because of the complexity of the central nervous system and the probable existence of a high affinity uptake system for the removal of the amino acid transmitters from the synaptic cleft, it has been technically difficult to provide evidence to satisfy the final criterion of a neurotransmitter, namely, demonstration of their release into the synaptic cleft with presynaptic stimulation. The use of in vitro superfusion of CNS tissue slices to study the release of radiolabeled neurotransmitter suspects has led to the following observations. The electrically stimulate release of amino acids is 1) specific for neurotransmitter candidates, 2) calcium dependent, 3) independent of tetrodotoxin-sensitive sodium influx, and, 4) influenced by carrier mediated exchange diffusion. This study proposes to extend these observations to the release of endogenous amino acid neurotransmitters using recently developed techniques for measurement of amino acids in picomole quantities. This will also facilitate investigation into the nature of the pool in the tissue slice from which the transmitter is released by allowing measurement and comparison of the specific radioactivity of the pools labeled by exogenously administered amino acid or by amino acid newly formed from radioactive precursors with that released by electrical stimulation. The location of the releasable pool of transmitter within the slice will be investigated further by measurement of the specific activity of the labeled amino acid in subcellular fractions. Finally, with the discovery of specific inhibitors of amino acid uptake, it should be possible to study the release of amino acids from cat spinal cord in vivo.